Recently, mobile electronic devices such as mobile audio devices, mobile phones, laptop computers and the like have been widely used, and various types of secondary batteries have been used as power supplies for such mobile electronic devices. Also, a demand for secondary batteries having a much larger capacity than is provided for the mobile electronic devices has been increased. For example, from the viewpoint of energy savings or reduction of carbon dioxide emission, hybrid vehicles using an electric driving power in addition to the conventional engine are becoming popular. For these reasons, secondary batteries having further improved characteristics of output, capacity, cycle life and the like are now desired regardless of the usage.
A secondary battery accumulates charges using an oxidation/reduction reaction. Therefore, a substance which is reversibly oxidation/reduction-reactable, namely, an electricity storage material which accumulates charges, greatly influences the above-described characteristics of the secondary battery. Conventional secondary batteries use metals, carbon, inorganic compounds and the like as the electricity storage materials. In the case of, for example, lithium secondary batteries widely used today, metal oxides, graphite and the like are used as positive electrode active substances and negative electrode active substances which are electricity storage materials.
In place of these inorganic materials, it is now being studied to use organic compounds as the electricity storage materials. Organic compounds allow more diversified molecule designs than inorganic compounds. It is considered that when an organic compound is used as an active substance, such an active substance can have various characteristics by molecule design.
Organic compounds are more lightweight than metals. Therefore, when a secondary battery is formed using an electricity storage material formed of an organic compound, the obtained secondary battery can be lightweight. For this reason, organic compounds are considered to be preferable for secondary batteries for hybrid vehicles, which do not need to have a high charging density but need to be lightweight. It has also been studied to use capacitors as electricity storage devices for hybrid vehicles. The above-described advantages of organic compounds are also provided when electricity storage materials formed of organic compounds are used for capacitors using a chemical reaction.
In Patent Documents Nos. 1 and 2, the present inventors have proposed an organic compound having a π-conjugated electron cloud as a novel electricity storage material which can provide high speed charge/discharge, and clarified a reaction mechanism thereof.